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The thermomechanical stresses due to mismatch of the coefficients of thermal expansion (CTE) of the base material (SiC) and the packaging has a significant impact on the stresses in MEMS pressure sensors used in high-temperature applications, to 600°C. The pressure sensor studied essentially consists of a SiC die attached to an AlN substrate using a gold die attach. Characterization of the stress distribution within the die attach, die and substrate along with the fatigue resistance of the die attach at 600°C is essential to estimating the reliability of the packaging structure. A parametric study has been performed using nonlinear finite element analysis to optimize the die-attach thermomechanical performance at high temperatures. This study includes the effects of varying porosity levels and varying reference temperatures (stress-free temperature). This study also provides information about the mechanical deformations of the pressure sensor due to the thermomechanical load, which must be compensated, for the effective performance of the pressure sensor. The outcome of the study provides guidelines to optimize the design of the pressure sensor.